Enzyme Mechanisms C483 Spring 2013.

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2 Questions1. Replacement of the amino acid ________ at or near an active site of an enzyme is more likely to change enzyme activity than the replacement of ________ at or near the active site.A) histidine; leucineB) leucine; histidineC) leucine; isoleucineD) histidine; aspartate2. The following pH dependence was found for the activity of a certain enzyme-catalyzed reaction. If it is known that the only two ionizable residues in the active site are both glutamates, which conclusion can be drawn?A) The glutamates have different microenvironments which cause their pKa's to differ.B) One of the glutamates must be amidated.C) Both glutamates have a pKa equal to 5.0.D) Both glutamates are deprotonated during the reaction.

3 3. An update of Fischer's lock-and-key theory of enzyme specificity views the ________ as the lock and ________ as the key.A) enzyme; substrateB) substrate; enzymeC) enzyme; transition stateD) transition state; enzymeE) substrate; transition state4. One reason the proximity effect enhances catalysis is becauseA) the effective molarity of reactive substrate groups increases.B) the enzyme changes conformation to more readily accept the substrate as it approaches the active site.C) the active site becomes smaller.D) the catalytic triad in the active site becomes more flexible.

4 MechanismsFour major mechanisms—any or all may be used in a given enzymeBinding MechanismsProximity effectTransition State StabilizationChemical MechanismsAcid-base catalysisCovalent Catalysis

5 Binding Energy Binding based on intermolecular forces “Lock and Key”SelectivityRate EnhancementEffective concentrationEntropy trapProductive orientation of two moleculesin the active site

6 Effective Molarity May be higher than actual molarity possibilityEntropic help

15 General Acid-Base CatalysisH+ and HO- are “specific acid/base” and depend on pHAmino acid sidechains are general acid-base, and can conduct reactions inside active site pocket that aren‘t possible in solution